Question: RS485 connection between 5V and 3.3V

Hi there I have a question about ADM2484.
Are there any issues if I use ADM2484 like the attached file?
(The master is ADM2484 with 3V operation and the slave is 5V operation such as ADM4581)

I'm a bit worried about the current flow from ADM4851 to ADM2484's VDD2pin when ADM4851 5V is Tx mode.

And one more thing, apparently both ADM2484 and ADM4851 have True fail safe mode.
Do you think it is okay to remove 1kohm pull-up and pull-down resistor?

Best regards,
Tarzan

QuestionnaboutADM2484.pdf
  • Hi Tarzan,

    The circuit that you have provided should operate correctly. I can't see any issues with it.

    Both the ADM2484E and the ADM4851 are fully RS-485 compliant, and operation in the bus setup that you have described should not be an issue.

    The ADM2484E/ADM4851 have the true fail safe receiver feature, which should eliminate the need for biasing resistors for an open or shorted bus state.

    However, if the customer would like to enhance the Noise Margin at the receiver inputs, then they can add the external biasing resistors.
    In your case the 1K Ohm biasing resistors will provide additional noise margin at the receiver input.
    The AN-960 application note pages 6 and 7 provides a description.
    http://www.analog.com/static/imported-files/application_notes/AN-960.pdf

    Regards,

    Richard

  • Hi Richard,

    Thanks for your reply.

    I checked out  AN-960 but I still don't  undertsand  the faill safe mode.

    I 'd like to make sure the advantages of this function.

    According to it, it says the basing resistors can be remobed if all RS485 IC have the true fale safe mode.

    But it looks like they have the undefined area between -0.03 to -0.2V.

    As you pointed oput the noise margine, I'm not sure if everyone benefits from this function.

    (I assume everyone use the biasing resistors just in case, there is no cost impact)

    Do you think removing registors brings any benefits to users?

    Best regards,

    Tarzan


  • Hi Tarzan,

    Using AN-960, page 6 (open fail-safe) with RT = 120, R = 1000, and Vcc = 3.3V:
    --> the biasing produces an offset voltage of 187mV.
    In the presence of noise around 0V, this 187mV offset will ensure that the RxD will still be at logic HIGH during the idle bus condition.

    Yes, the biasing resistors can be removed from bus if the RS-485 transceiver has true fail safe mode.
    For the ADM2484E the customer does not need biasing resistors for an idle bus condition failsafe.
    However, adding the biasing resistors will provide additional noise margin (as described above).
    Some customers use biasing resistors, and some don't use the biasing resistors. It depends on their application environment.


    Regards,

    Richard

  • Hi Richard,

    Thanks for your support. I really appreciate it!!

    I understand what you mean.

    Please let me ask if there another question comes up.

    Best regards,

    Tarzan